Three dimensional electron beam treatment planning evaluation for post op radiotherapy of the chest wall

Abstract

A multi-institutional collaborative working group (CWG) has conducted a study to evaluate electron beam three dimensional treatment planning for post-operative radiotherapy of the chest wall. This included defining criteria for target volume, dose uniformity, the use of bolus and the requirements for incorporating tissue inhomogeneities in treatment planning. Patients for this particular disease site were selected so that they would provide the opportunity to study the following treatment planning challenges: (1) sloping surface topography, (2) underlying inhomogeneities (lung), (3) varying thickness of chest wall over lung, (4) field matching techniques, (5) bolus effects on dose distribution and (6) electron arc dosimetry studies. Patients entered into this study had histology of invasive carcinoma of the breast (all subtypes) and were clinical stage T3 and T4 breast cancer (postmastectomy). Patients with positive margins and inflammatory breast were excluded. Patient CT data sets were obtained under protocol conditions. This provided a contiguous set of CT slices 10 mm thick from 5 cm superior to the sternal notch to 5 cm inferior to the xyphoid. The protocol called for a dose of 4500 cGy to the supraclavicular volume, 5000 cGy to the internal mammary volume and the postmastectomy chest wall. Critical structures evaluated included the lung, heart, and skin. Plans were done with a lD, 2D, and 3D dose calculation algorithm with the patient CT data sets modified to be water equivalent and then the plans were recalculated using the CT tissue data. Finally, a best 3D plan was developed using non-standard beam arrangements, e.g. non-coplanar beams and any and all 3D planning tools available. Plans were then evaluated under a criteria established by the CWG and compared to calculated tumor control probability and normal tissue complication probabilities. Full details of the study for this particular site will be presented.